1. Field of the Invention
The present invention relates to an image encoding apparatus and its method such as JPEG-2000 to compress images by means of wavelet transform and entropy coding.
This application claims priority of Japanese Patent Application No. 2003-316092, filed on Sep. 8, 2003 and Japanese Patent Application No. 2003-367809, filed on Oct. 28,2003, the entireties of which are incorporated by reference herein.
2. Description of the Related Art
Conventionally, representative image compression techniques include JPEG (Joint Photographic Experts Group) standardized by ISO (International Standards Organization). It is known that the JPEG technique uses Discrete Cosine Transform (DCT) and provides excellent encoded and decoded images if relatively high bits are assigned. However, if the number of encoding bits is decreased to an excess, DCT specific block distortion becomes remarkable and subjective deterioration becomes conspicuous.
In recent years, extensive research is carried out on techniques that divide an image into a plurality of bands using a filter bank, i.e., a combination of a high-pass filter and a low-pass filter, and encode each of bands. Among these techniques, a special attention is paid to wavelet transform coding as a new technology that takes the place of DCT. The wavelet transform coding is free from a DCT drawback that causes remarkable block distortion in high compression.
The JPEG-2000 technique completed international standardization on January 2001. JPEG-2000 combines the wavelet transform with highly efficient entropy coding (bit-plane based bit modeling and arithmetic coding). JPEG-2000 more greatly improves coding efficiency than JPEG.
The international standards specify only decoder standards and permit design freedom to encoders. By contrast, there is no standard as to efficient rate control techniques to realize targeted compressibility ratios. The greatest importance is attached to establishment of know-how. JPEG makes the rate control especially difficult It has often been necessary to perform coding more than once until a targeted value is reached. However, this causes an increase in the processing time. JPEG-2000 is expected to obtain a targeted code amount through one-time encoding.
Generally, JPEG-2000 employs the rate control technique using RD (Rate-Distortion) characteristics. This rate control technique excels in versatility but has a drawback of causing very high calculation load.
As described in patent document 1 (Jpn. Pat. Appln. Laid-Open Publication No. 2002-165098) shown below, the applicants propose the technology of providing rate control by truncating a generated encoded code stream from the end. This technology enables control accurately adjusted to a targeted code amount.
However, the technology in patent document 1 may degrade the image quality by eliminating a code block from the end of an encoded code stream if the code block has a great influence on the image quality. It is expected to provide a more efficient rate control technique.
Generally, JPEG-2000 has no standard as to techniques of decreasing highly loaded entropy coding. The greatest importance is attached to establishment of know-how.